1. Field of the Invention
The present invention relates to a disc antenna which consists of a conductor disc and a conductor ground plate placed in parallel to each other and which radiates non-directional electric waves from the space between them.
2. Description of the Prior Art
In recent times very remarkable developments have been made with respect to underground streets and towns. As to radio communication systems required for fire-fighting activities, police activities and others in such confined places, a system has been employed in which leaky coaxial cables are laid in the underground streets. However, that system has shortcomings in that it is very expensive to construct a close network of long and large cables in the underground street and also in that it is detrimental to the appearance of the streets.
A system has also been employed in which unipole antennae or the like are installed at fixed intervals in the interior of the underground streets, and these antennae are fed by means of cables connected to their feeding terminals above ground. In the case of this system, however, it involves such problems in that the positions required for the installation of the unipole antennae have to be provided and that, where the underground streets or the like have low ceilings, the antennae may be found a nuisance or may be of a weak construction that is readily damaged.
Furthermore, it has to be noted on the other hand that, although the frequencies in use at the present time for short-distance communication radios concentrate in the 150 MHz band, a change-over to the 400 MHz band in the future is considered possible. It is, under these circumstances, necessary to have antennae common for the two frequency bands of 150 MHz and 400 MHz. The antennae that have been described above will be found unsuitable when a change in frequency is made, or when a multiple frequency service is adopted.